How to Start Tomatoes From Seed

BENEFITS OF STARTING YOUR TOMATOES FROM SEED

Growing your own tomato plants from seed will not only save you money but can also be wildly satisfying. If you love to garden but find yourself getting the winter blues, I highly recommend starting your garden from seed. It will give you extended time to tend to your garden before your weather is ready. 

You will open up a new world of tomato varieties by starting your plants from seed. Your local garden centers are a great place to get seeds, but they tend to be more limited in varieties than what you can find when shopping for seeds online. You will want to be sure you are ordering from a reputable company for your seeds. The germination rates will be higher, and you will also feel more confident that what you are tending to for months will be true to what you expect. 

WHAT DO I NEED TO START TOMATO SEEDS INDOORS?

You will need a handful of items to start your adventure of starting your plants from seed. 

• Seeds. 
• The varieties depend on what tomatoes you want to grow.

• Grow lights
• A light source is essential when starting seeds. You can start some plants on a windowsill, but most plants need more light than what our window sills can offer in the winter and early spring. LED lights made for growing mimic daylight and are best for seedlings. 

• Seed-starting soil. 
• You can mix your own or purchase a seed starting mix. Do not use garden soil as it is too heavy, tends to have poor drainage, and the seeds have a hard time competing with the debris in garden soil. 

• Seed starting containers.
•  You can use these 6 cell trays for various plants to save space and still have healthy transplants. They are very heavy-duty so that you can reuse them. 
• Up potting containers. 
• I use these 32-cell reusable plant starter trays with inserts to pot up my tomato and flower starts. 
• If you live in a colder climate or want larger transplants, you can do a second round of up-potting into 3.3-inch pots or even 5-inch pots. 
• Related: How to Transplant and Up Pot Seedlings
• Seed-starting heat mats help speed up germination and are designed to reliably warm the root zone by 10-20⁰F above ambient air temperatures. 
• 1020 extra strength trays 
• Proper bottom trays make bottom watering and carrying plants in and out for the hardening-off process much more manageable. 

• This small-batch seed starter kit has everything you need in one convenient kit. LED lights, heat mat, 1020 trays, and 2.5" pots 

WHAT IS THE DIFFERENCE BETWEEN HEIRLOOM TOMATOES AND OPEN-POLLINATED TOMATOES?

Heirloom and open-pollinated terms are terms you will run across when looking for seeds. To better understand them, here is a quick definition of each one. 

Heirloom seeds: the seed has been growing unchanged for 50+ years reliably. You can save seeds from them and expect to grow the same type of plant. 

Open-pollinated: the seeds are produced by natural pollination of the parent plant either by birds, bees, self-pollinating, or other insects. There are some very neat varieties of plants that have been produced with this method. 

WHAT IS THE DIFFERENCE BETWEEN ORGANIC TOMATOES AND HYBRID TOMATOES? 

You will often see organic or hybrid when you are searching for seeds. Don't confuse hybrid with GMO; this is not the same thing. 

Organic tomato seeds: seeds produced by certified organic gardening and farming methods. The mother plants have been grown without synthetic fertilizers or pesticides. 

Hybrid tomato seeds: seeds created by crossing two or more different but related varieties. This is usually done to improve flavor, productivity and to help make a plant more disease and pest-resistant. Usually considered unstable, saving seeds from a hybrid plant will not produce the same as the parent plant. This seed could either revert to one of the grandparent's traits or not produce at all even.

WHAT IS THE DIFFERENCE BETWEEN DETERMINATE AND INDETERMINATE TOMATOES?

When talking about tomato plants, there are two different growth habits that you should be familiar with: determinate and indeterminate. 

Determinate varieties grow to a determined height and set most of their flowers and fruit at relatively the same time. The fruit tends to ripen all at once, and then the plant is done. These are good for making sauces because you will get a flush of fruit all at once in a short period. 

Indeterminate varieties will keep growing and setting flowers and fruit until disease or frost takes them out. The fruit ripens throughout the summer, giving you fresh tomatoes all season long.

HOW MANY TOMATO PLANTS SHOULD I GROW?

First, you will need to map out your available space. Most large plants like tomatoes require 12-24 inches between plants. This is also dependent on if you plan on pruning or not. Pruned plants can be planted closer together. 

Trellising also allows you to plant your tomatoes much closer together. Make use of vertical space to get far more tomatoes out of the same square footage of garden space. There are many good ways to support growing tomatoes effectively; traditional tomato cages, trellis netting, or roller hooks with vine clips allow you to grow up instead of out. For greenhouse growing, using the lower and lean method is the most effective for growing large numbers of cherry tomatoes or small sauce vartities.   

Now you need to think about your plans for the tomatoes you are growing. Are you planning on only eating them fresh? If so, you can easily plant 1-2 plants per person. Are you hoping to preserve them and make spaghetti sauce, marinara, salsa, or tomato soup? Then you will want to grow even more. It takes about 20 lbs. of fresh tomatoes to prepare 7 quarts of sauce.

I always plant a few more plants than what I think I'll need just in case I lose a plant or two before it reaches maturity. If you find yourself with too many, you are sure to find a neighbor or family member who would gladly take them. 

You can expand your growing space by growing in grow bags also. I've grown tomatoes, carrots, squash, herbs, and even melons in grow bags and had great success. Grow bags are especially great for determinate varieties. 

WHEN DO I START MY TOMATO SEEDS?

You will first need to know your estimated last frost date. You can get this information by simply searching "last frost, your city and state." Remember, this is only an estimate, not a guarantee. You may get a late frost, so allow a little wiggle room in your planning. 

Once you have your last expected frost date, you will count backward the weeks to find the time you will want to start your seeds. Tomatoes can be started 6-10 weeks before your last expected frost. 

HOW DO I START TOMATO SEEDS?

Always moisten your seed starting mix before planting seeds. Seed starting mix often comes dry and needs to be hydrated. You want to be able to take a handful and form a ball without the soil being so wet that it wrings water out. Just moist enough to hold shape. 

Fill your container with soil and pack down slightly, leaving about ½ inch to the top. Place 1-2 seeds per container or seed starter cell—cover seeds with ¼ inch of soil. Keep soil moist during germination. 

WHAT SIZE CONTAINER DO I NEED TO START TOMATO SEEDS?

This decision varies depending on how long the plant will need to be in the tray or potted up before transplanting outside. 

When I start my tomatoes and peppers, I start them in 128-cell seedling trays. Each cell measures 1" square by 2¼" deep. Using a cell tray lets me plant many seeds and save space under the grow lights and on top of the heat mat. 

Once the seeds have germinated and two sets of true leaves, I pot them into 2.5" seed starting pots. The very first leaves that a plant has are called cotyledons. These are not considered true leaves and are an entirely different shape than adult leaves. They provide nutrients until the seedlings' true leaves emerge and begin the process of photosynthesis. 

Tomatoes benefit from being planted deep. When I pot them up, I bury them as deep as possible, leaving only the top ⅓ of the plant above the soil. I will also do this again when I plant them in the garden. This helps them grow a bigger root system. All the fuzzy hairs along the stem of the tomato plant will form roots. 

If you do not need as many cells to start your seedlings, you can start them in 6-cell plug trays or 72-cell seed starting trays. Starting tomatoes in smaller cells and then potting them up not only saves space it actually gives you a stronger plant when it comes time to transplant outdoors.  

SHOULD I WATER TOMATOES FROM OVERHEAD OR BOTTOM WATER?

Bottom watering is my the preferred method. It allows the soil to act as a wick and take up the water needed without displacing the newly sowed seed. I prefer to use 1020 trays underneath my pots and cell trays when starting seeds. I fill the tray about 1" - 1 ½" with water, let the soil soak up the water, and drain out any excess water after about an hour. 

Bottom watering is also time-saving and helps prevent damping off and other damage to your tender seedlings. Watering from the top can lead to a host of fungal issues, and tomatoes hate having wet leaves. 

When the seed first tries to germinate, you will want to ensure the soil is moist. Dry soil will prevent germination and stunt or kill off young seedlings. If the top layer of your soil stays dry when you bottom water, you can use a fine mist spray bottle to dampen the very top layer of soil during germination. 

HOW TO USE A HUMIDITY DOME FOR TOMATO GERMINATION 

Humidity domes increase germination success when used correctly - especially when used with a heat mat at the same time. Humidity domes act as a mini-greenhouse, trapping the moisture and heat in. Tomatoes prefer temperatures between 75-85 degrees for consistent germination.

Keep the humidity dome on until you see the first signs of germination. Once the seeds have germinated, uncover and ensure the newly germinated seedlings are under a light source. You can do this in stages if your growing space is colder than you'd like or has very low humidity. 

While waiting for the first true leaves to come in, you can leave the humidity dome on and open the vents to provide some airflow. It is crucial to remove the dome once the plants have a little height to them because if the dome is left on for an extended period, mold growth becomes more likely. 

HOW TO STRENGTHEN TOMATO SEEDLINGS 

Tender tomato seedlings grown indoors need a little help getting stronger before they are ready to be outside. Adding a fan to your growing area will help strengthen their stems. It will also improve air circulation and ventilation, helping to prevent mold and bacterial growth. 

HOW TO HARDEN OFF TOMATO PLANTS

It's time to start introducing your seedlings outside once your temperatures have warmed up and you are no longer at freezing temperatures during the day or night. 

If you have grown your seedlings indoors or in a greenhouse, you will slowly need to acclimate these tender plants to the outside world. This process is called hardening off. It takes about a week to do this properly. You do not want to skip this step. 

Remember your seedlings have been growing in a controlled environment. The temperature has been roughly the same day and night. They have not been exposed to the sunlight, which is stronger than grow lights. Tender seedlings are susceptible to sunburn, wind, rain, and temperatures fluctuations. 

Slowly bring your seedlings outside to acclimate them to the growing conditions. Increase the time each day until your seedlings have been outside day and night for a few nights. Choose calm days to start this process. Too much wind or rain will damage your tender plants. 

• Days 1-2: bring your trays of seedlings outside for 1-2 hours and place them out of direct sunlight in a protected area from wind and rain
• Day 3: increase the time outside by another hour, place them in an area with dappled sunlight. 
• Day 4: add another hour to the time the seedlings are outside (they should be outside now about 4 hours) 
• Day 5-7: they are now able to tolerate longer periods of time, as well as more direct sunlight.

Around 7 days, your plants should be prepared and hardened off. You will notice they are a bit tougher and may have become a deeper green. As long as your forecast looks good and your temperatures are staying well above freezing, you can now leave your baby plants out overnight. I personally like to hold off on planting them in the ground until they've spent a few full around-the-clock days outside. 

GROWING YOUR OWN PLANTS FROM SEED 

The vast array of tomato seeds available will make growing your plants from seed so fulfilling. Another added benefit is having complete control over when your transplants will be ready and not relying on the stores to bring in plants.

Being able to grow not only red tomatoes but multicolored, striped, white, purple, orange, black, and green tomatoes will bring life to whatever you are serving. Even a simple salad will look extraordinary with all the different colors you can produce from your home garden.

You can save in the long run because the supplies you buy will be able to be reused for years to come, and you will only need to acquire some seed-starting soil and some new seeds. You can even save seeds from your favorite plants and build up a seed stock of specialty climatized seeds for your specific microclimate.

 Written by Robin Lapping, @robinsroots

Updated by Emily Gaines, March 2024